The work proposed in this application is aimed at developing techniques to increase the rate of production of macaques with desirable genotypes. Initially attention will be given to the use of controlled multiple ovulation and embryo transfer into surrogate females. Multiple embryo transfers into individual surrogates to increase the efficiency of the procedure will be studied. This work will provide information on whether macaques can successfully bear twins. The application also proposes the development of techniques to produce macaque artificial monozygotic twins by bisection of preimplantation embryos. Before attempting to produce these animals it is important to test the effect of the operation on the phenotypic variability within the twin offspring. This question is most easily examined in mice. If bisecting mouse embryos does not increase this variability, and if we show that macaques can bear twins, we will then attempt to produce macaque artificial monozygotic twins. Even if macaques cannot bear twins and/or if bisection of embryos increases intratwin variability the production and transfer of half-embryos will be attempted in order to increase the number of desirable genotypes.